The FAA had originally planned to re-open US airspace to civil services at noon (1600 GMT) on Wednesday.

But the deadline passed without the ban being lifted, and there is still no timetable for a full resumption of services.

An estimated 4,000 commercial aircraft, almost one third of the world's fleet, were grounded in the US shortly after Tuesday's attacks on the Pentagon and World Trade Center.

New safety measures

The US government has issued new safety guidelines to airlines and airports.

The measures include armed plainclothes guards travelling on flights, all passengers having to check in at ticket counters only, and boarding areas will now be restricted to just passengers.

Also, no knives of any kind will be allowed on board planes.

The knock-on effect

The devastating terrorist attacks on the US have plunged airports around the world into chaos, with widespread congestion and delays expected to last for days.

And after what has been called "the blackest day aviation has ever had", the industry is facing severe financial consequences.

Yesterday's events will have a catastrophic effect on world aviation

Phil Butterworth-Hayes, Aviation expert

Airports around the world are already filled to bursting with stranded passengers after flights to the US were halted or diverted on Tuesday.

And further cancellations on Wednesday - including British Airways flights to Canada, the US, Israel and Islamabad, Pakistan - will aggravate the congestion, causing a spiralling effect to delays.

A return to normality is expected to be a lengthy process, due to the need for extremely tight security measures and the lack of space for aeroplanes on the ground.

And even once airports do reopen, stringent security of a kind unfamiliar to most US domestic air travellers is likely to become the norm.

International standstill

Many flights were grounded earlier on Wednesday at the international airports of Japan, Hong Kong, Singapore, China, Taiwan and South Korea.

US flights make up 25% of business for some major Asian airlines, according to Salomon Smith Barney airline analyst Tiffany Co.

Among European carriers, British Airways said it had cancelled all US flights for the second day in a row, as well as halting flights to Islamabad in Pakistan, because of its proximity to Afghanistan.

Security has been tightened at London's Heathrow - the world's busiest international hub - which normally has 68 scheduled flights a day to the US.

On Tuesday, the UK government banned all commercial air traffic from flying over the capital, London, as did Belgian authorities with planes due to pass above Brussels.

Airspace in Israel remained closed.

And even airways which do not fly to the US, such as the UK's EasyJet, are being affected.

The low-cost carrier has been forced to cancel flights because of the sheer volume of people at airports, and "to prevent spiralling delays".

Return to normal

Getting the world's airlines back to normal could be a lengthy process.

Many planes will be in the wrong place since many planes were diverted elsewhere on Tuesday, especially Canada.

Later on Tuesday, Canada's government closed Canadian airports - now full to bursting with stranded passengers - to all aircraft except incoming flights already diverted from the US and "those flights required for humanitarian purposes".

Under normal circumstances a large proportion of the world's planes are in the air at any point in time.

Space at airports is scarce, so landing planes within a narrow timeframe presents huge logistical difficulties.

Airline shares fall

And while the industry is desperately trying to solve the short-term problems, it will face a whole host of new difficulties in the longer term.

Traders were some of the first to react to the terrorist attacks, and shares in airlines another firms involved in the travel industry have fallen sharply

"Yesterday's events will have a catastrophic effect on world aviation," said Phil Butterworth-Hayes, civil aviation editor at Jane's Information Group.

The industry will also foot the bill for new measures that will be introduced in an attempt to prevent such a disaster happening again.

And some airlines will choose to pay passengers compensation for the long delays and cancellations.